vsprintf: Handle NULL with %pU

At present a NULL pointer passed to printf for a %pU argument will cause
U-Boot to access memory at 0. Fix this by adding a check, and print
"(null)" instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexander Graf <agraf@suse.de>
[agraf: s/(null)/<NULL>/]
Signed-off-by: Alexander Graf <agraf@suse.de>